I doubt that the tank smells so bad like ammonia with the fishes still alive in it. Still, change about 50% of the water today and then again 50% tomorrow to be on the safe side. And get a test kit as soon as possible, so you will see now and in the future (your tank still has to cycle) how high the water parameters (ammonia, nitrite, nitrate) are. Your fishes can die quickly if the ammonia or nitrite levels are too high and then they might just be swimming with their bellies up one morning.
Did you read anything about cycling a tank with fish in it yet?